This invention relates to a two-speed power transmission for a power tool, like for instance a screw joint tightening tool.
In particular, the invention concerns a power transmission which comprises an input shaft, an output shaft, a planetary gear, and a torque responsive gear shift mechanism for directing torque through the planetary gear in a high torque/low speed drive mode or past the planetary gear in a low torque/high speed drive mode. The planetary gear includes a sun wheel connected to the input shaft, a ring gear and a planet wheel carrier, and the gear shift mechanism comprises a driving member connected to the sun wheel of the planetary gear, a driven member connected to the output shaft, and a number of coupling elements arranged to intercouple in a first position the driving member, and the driven member and to intercouple in a second position the planet wheel carrier and the driven member.
A two-speed power transmission of the above type is previously known through EP-B1-0136991. A drawback inherent in the device described in this publication concerns the gear shift mechanism which comprises radially acting coupling elements. This feature is disadvantageous in two different aspects, namely its tendency to cause an undesirable enlarged outer diameter of the power tool, and its dependency of rotational speed since the centrifugal action influences upon the radial shifting movements of the coupling elements and, accordingly, upon the drive mode shifting point of the transmission.